1. Field of the Invention
The present invention relates to a waveshaping circuit for an analog signal, and more specifically to a circuit which shapes an analog signal which should have the same peak value (absolute value) for positive and negative peaks, so that the positive and negative peak values are the same. Yet more specifically, the present invention relates to a circuit which detects and corrects the positive/negative asymmetry of an analog waveform with respect to a proper reference level for the analog signal, considering the condition in which there is a DC offset component in the reference level of the analog signal, so that the positive and negative peak values of the waveform are the same.
2. Description of Related Art
In the past, when recording data or when reading data using an AC analog signal, processing was facilitated if the amplitude with respect to a reference level exhibited positive/negative symmetry. Stated differently, signal processing is easier when the positive-side and negative-side peaks of the analog signal with respect to the reference level are the same. In a magnetic disk, a playback signal read by using an inductive head has positive-side and negative-side peak values with respect to the reference level that are almost the same. In contrast to this, when reading data using a magnetic resistance effect head (hereinafter referred to as an MR head), the positive and negative peaks with respect the reference level differ. As a result, the configuration of the circuit used to demodulate the playback signal from an MR head becomes complex. Therefore, there is a desire for a magnetic disk apparatus using an MR head which enables simplification of the configuration of the circuit used to demodulate the playback signal from the MR head.
In recent years, with an increase in the speed of computer systems, there have also been demands for high speed and large capacity with respect to magnetic disk apparatuses used as external storage devices. For this reason, the frequency of the signals processed in the demodulation circuit of magnetic disk apparatuses have increased, with an accompanying increase in the density of the recording onto the recording medium. In magnetic disk apparatuses in the past, while a single thin-film head was used for both recording and playback, because a thin-film head uses the method of detecting the amount of change of magnetic flux per unit time of the rotating disk, near the center of the disk, at which the speed of the disk is slow, it is difficult to read data, making it difficult to achieve a large capacity.
Because of the above-noted situation, to improve the signal quality when performing high-density recording, a compound MR head, in which a head dedicated to reading of data from the disk is combined with the thin-film head of the past, has reached the stage of practical use. Because data is read the MR head only by the strength of the magnetic flux of the disk, by using the MR head it is possible to read data from the entire disk surface.
However, because of the low conversion efficiency of an MR head, while it has advantages such as a large signal output when performing high-density recording, the signal waveform often has positive and negative peak values with respect to the signal reference level which are not equal. For this reason, when demodulating a signal played back using an MR head, which has positive and negative peak values that mutually differ, the problem of the occurrence of a large number of demodulation errors occurs, this causing a worsening of the error rate.
Also, unless the signal path in the demodulation circuit is directly linked in the DC sense, an analog signal read using an MR head and which has positive and negative peak values that are different is transmitted to the demodulation circuit as a signal waveform which includes a DC offset component, so that what should remain as the proper reference level is corrupted by the DC offset level, making it difficult to perform correction when the positive and negative peak values differ.